The present invention relates generally to film cooling holes for cooling airfoils in gas turbines, such as rotor blades and stator vanes, and more particularly to a new mini-trench shaped (MTS) cooling hole.
Airfoil surfaces inside gas turbines, such as jet engines, have to survive very high temperature environments. A primary method for protecting those surfaces is by including cavities inside the airfoils so that cooler air from other gas turbine sections, typically compressor air, can be supplied to those cavities and then discharged through small openings, or film cooling holes, in the airfoil surface to form a protective boundary layer, or film, of cooler air.
Modern production gas turbine engines typically use two types of film cooling holes: cylindrical, constant-diameter holes; and, laterally-expanding, forward-diffused, laidback fan shaped holes, part of a group of so-called shaped holes.
Cylindrical holes typically work best for leading edges of airfoils. Shaped holes typically work best for the remaining surfaces.
U.S. Patent Application Publication 2013/0115103 by Dutta et al., in its FIG. 1, shows an example row of laidback fan shaped holes 12.
The Dutta et al. patent application also shows a modification to laidback fan shaped holes 12 by adding a small trench 14 inside each shaped hole.
U.S. Patent Application Publication 2011/0097188 by Bunker shows a series of shaped holes inside a shallow trench.
Such trenched shaped holes appear to improve cooling by providing space beneath a main flow for providing additional cooling air as needed.
Trenches especially help with lateral cooling because they increased cooling effectiveness between individual cooling holes as that cooling air is spread over the length of a row of shaped holes without increasing demand for mass flow of air. Many proposed trench configurations have been tested in recent years and the best performing configurations so far involve shaped holes inside a trench.
A problem with such shaped holes inside a trench is that they are difficult to manufacture. The forward edge of the trench, for example, blocks an electrical discharge machining (EDM) drill from making the complicated shapes of a shaped hole inside.
There is, therefore, a need for an improved film cooling hole configuration that provides the same, or better, performance as shaped holes inside a trench, and is also easier to manufacture.